Mass spectrometry is commonly used to analyze trace substances. In such analysis, firstly ions are produced from the trace substance to be analyzed. As shown in FIGS. 13 and 14 of U.S. Pat. No. 4,328,420 to J. B. French, such ions may be directed through a gas curtain into an AC-only set of quadrupole rods. The AC-only rods serve to guide the ions into a second quadrupole rod set which acts as a mass filter and which is located behind the AC-only rods. The AC-only rod set also separates as much gas as possible from the ion flow, so that as little gas as possible will enter the mass filter. The AC-only rods therefore perform the functions both of ion optic elements and of an ion-gas separator.
In the past, it had been believed and the evidence has shown, that ion transmission through ion optical elements including AC-only rods and through a small orifice at the end of such optical elements, increases with lowered gas pressure in the ion optic elements. For example the classical equation for a scattering cell shows that the ion signal intensity (ion current) transmitted through the cell decreases with increasing gas pressure in the cell. Unfortunately the resultant need for low pressures in the region of the ion optic elements has in the case of gassy ion sources required the use of large and expensive vacuum pumps. This greatly increases the cost of the instrument and reduces its portability.
The inventors have now discovered that the classical equation describing ion signal intensity does not in fact describe the situation accurately when dynamic focussing is used in the interstage region and that when the gas pressure in the region of the ion optic elements is increased within certain limits and when the other operating conditions are appropriately established, ion transmission is markedly increased. The reasons for this are not fully understood but the effects in some cases are dramatic. In addition, when such increased pressures are used under appropriate conditions, as will be described, focussing aberration of the ion optics is reduced. In addition the ion energy spreads are reduced.
In one of its broadest aspects the invention provides a mass spectrometer system comprising:
(a) first and second vacuum chambers separated by a wall, said first vacuum chamber having an inlet orifice therein, PA1 (b) means for generating ions of a trace substance to be analyzed and for directing said ions through said inlet orifice into said first vacuum chamber, PA1 (c) a first rod set in said first vacuum chamber extending along at least a substantial portion of the length of said first vacuum chamber, and a second rod set in said second vacuum chamber, each rod set comprising a plurality of elongated parallel rod means spaced laterally apart a short distance from each other to define an elongated space therebetween extending longitudinally through such rod set, said elongated spaces of said first and second rod sets being first and second spaces respectively, said first rod set being located end to end with said second rod set so that said first and second spaces are aligned, PA1 (d) an interchamber orifice located in said wall and aligned with said first and second spaces so that ions may travel through said inlet orifice, through said first space, through said interchamber orifice, and through said second space, PA1 (e) means for applying essentially an AC-only voltage between the rod means of said first rod set so that said first rod set may guide ions through said first space, PA1 (f) means for applying both AC and DC voltages between the rod means of said second rod set so that said second rod set may act as a mass filter for said ions, PA1 (g) means for flowing gas through said inlet orifice into said first space, PA1 (h) means for pumping said gas from each of said chambers, PA1 (i) the pressure in said second chamber being a very low pressure for operation of said second rod set as a mass filter, PA1 (j) the product of the pressure in said first chamber times the length of said first rod set being equal to or greater than 2.25.times.10.sup.-2 torr cm but the pressure in said first chamber being below that pressure at which an electrical breakdown will occur between the rod means of said first rod set, PA1 (k) and means for maintaining the kinetic energies of ions moving from said inlet orifice to said first rod set at a relatively low level, whereby to provide improved transmission of ions through said interchamber orifice. PA1 (a) producing outside said first chamber ions of a trace substance to be analyzed, PA1 (b) directing said ions through an inlet orifice in an inlet wall into said first space, and through said first space, said interchamber orifice and then through said second space, and then detecting the ions which have passed through said second space, to analyze said substance, PA1 (c) placing an essentially AC-only RF voltage between the rod means of said first set so that said first rod set acts to guide ions therethrough, PA1 (d) placing AC and DC voltages between the rod means of said second rod set so that said second rod set acts as a mass filter, PA1 (e) admitting a gas into said first chamber with said ions, PA1 (f) pumping said gas from said first chamber to maintain the product of the pressure in said first chamber times the length of said first rod set at or greater than 2.25.times.10.sup.-2 torr cm but maintaining the pressure in said first chamber below that pressure at which an electrical breakdown would occur between the rods of said first set, PA1 (g) pumping gas from said second chamber to maintain the pressure in said second chamber at a substantially lower pressure than that of said first chamber, for effective mass filter operation of said second rod set, PA1 (h) and controlling the kinetic energy of ions entering said first rod set to maintain such kinetic energy at a relatively low value;
In another of its broadest aspects the invention provides a method of mass analysis utilizing a first rod set and a second rod set located in first and second vacuum chambers respectively, said first and second rod sets each comprising a plurality of rod means and defining longitudinally extending first and second spaces respectively located end-to-end with each other and separated by an interchamber orifice so that an ion may travel through said first space, said interchamber orifice and said second space, said method comprising:
whereby to provide improved transmission of ions through said interchamber orifice.
Further objects and advantages and advantages of the invention will appear from the following description, taken together with the accompanying drawings.